The present invention generally relates to aqueous pigmented coating formulations for improving opacity in paper, paperboard, and related products and methods for making such aqueous pigmented coating formulations. More particularly, the present invention relates to aqueous pigmented coating formulations comprising an acylated lecithin for use in coating paper and paperboard. The acylated lecithin increases the opacity of paper and/or paperboard coated with the aqueous pigmented coating formulation.
Aqueous pigmented coating formulations are generally used in processes for making paper and paperboard. The unique functional and optical properties of paper are derived through the paper making process and the coating formulations used therein. Conventional coating formulations are dispersions of mineral pigments, binders, and additives in water. Pigment-containing coating formulations provide paper with a desirable finish, gloss, and smoothness. For example, one major function of coating formulations, such as the aqueous pigmented coating formulations, is to improve the appearance of the paper or paperboard by increasing properties such as opacity, brightness, and surface smoothness and gloss. Typically, the pigment in the coating fills in irregularities in the paper surface, producing an even and uniformly absorbent surface for printing and improving the overall appearance of the coated sheet.
The binder or adhesive present in the formulation influences the properties of the coating formulation and the properties of the final coated paper. For example, the binder functions: (1) to impart the required rheology behavior and water retention to the coating formulation; (2) to bind the pigment particles together in the dried coating and to the paper web; and (3) to control the absorption of printing ink during printing on the paper.
The opacity of paper is that property of paper which minimizes the show-through of light through the sheet or printing from the back side or the next sheet. The opacity of a surface coating on paper and paperboard is particularly important where the base fiber is dark or non-uniform. Opacity may be affected by thickness, filler, and calendering of the coating formulations used in the papermaking process. Specifically, opacity is derived through a combination of light scattering and light absorption within the coating layer of coated paper or coated paperboard. Generally, a high level of opacity in the coating layer and in the finished paper or paperboard structure is desirable to improve the quality of the finished product.
There have been several attempts at increasing the opacity-providing characteristics of aqueous pigmented coating formulations for use in the papermaking process. For example, high refractive index pigments such as titanium dioxide are well-known to increase opacity, but these materials are costly and affect other properties, such as color and shade, which may not be desirable for some paper grades. Additionally, titanium dioxide has high ultraviolet absorbance, which negatively impacts the optical brightening efficiency of fluorescent dyes added to the coating as florescent whitening agents.
Other approaches to increasing opacity focus on improving the light scattering. This includes approaches, such as utilizing the mechanism of increased void volume and the consequent increase in the number of air-pigment interfaces, to promote additional scattering, or using structured pigments, such as anhydrous calcined clays. These structured pigments are more expensive than conventional pigments. Additionally, they usually increase the abrasivity of the coating formulation, leading to faster wear on the production equipment, with consequent costs for downtime and maintenance of replaceable wear elements like coater blades. Structured pigments can also impact the print quality of paper, as the higher capillary volume of the void-filled pigment can lead to rapid ink setting and resulting defects due to ink mottle or non-uniform ink distribution in the printed area.
Light scattering, and its impact on opacity, has also been utilized to increase opacity through the use of cationically-charged polymers which flocculate the anionic pigment slurry to give random pigment packing and consequent higher light scattering. These materials are generally difficult to control in practice since the operational efficiency of the coating is often compromised by the addition of flocculants, and small dosage variations can result in major issues with stability. As such, destabilization of the colloidal pigment suspension is not a desirable route to higher scattering.
Light absorption, such as that obtained by adding black dye to the coating formulation, is another well-known route to obtaining higher opacity in the finished product. Through the proper selection of dye colors, which can include, for example, blue, black, and red, the final shade of the coating may be maintained, although the use of dyes that absorb in the visible region may reduce the brightness or overall reflectance of the coating. This usually results in an unacceptable compromise in the final sheet quality of the coated paper.
As such, a need exists in the industry for an aqueous pigmented coating formulation that provides an increase in the opacity of paper and/or paperboard coated with the aqueous pigmented coating formulation. Additionally, it is desirable that the increased opacity is obtained without compromising the other physical or optical properties of the paper or paperboard and by avoiding the use of expensive additives such as high refractive index pigments.